Devulcanization of rubber and other elastomers

ABSTRACT

A devulcanization apparatus for devulcanizing a plurality of cross-linked elastomer particles. The apparatus includes a first conveyor functioning as a high voltage electrode and a second conveyor functioning as a ground electrode. A generator is operable to apply an alternating electric field between the first and second conveyors. A devulcanization region is provided between the first and second conveyors in which the cross-linked elastomer particles are placed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a divisional of U.S. patentapplication Ser. No. 13/185,167, filed on Jul. 18, 2011, now U.S. Pat.No. 8,470,897, which is incorporated herein by reference in itsentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the recycling of vulcanized elastomers,such as vulcanized rubber found in tires, involving the use of analternating electric field to devulcanize the elastomers.

2. Description of Related Art

Millions of used tires, hoses, belts, and other vulcanized rubberproducts are discarded annually after they have been worn-out duringtheir limited service life. These used vulcanized rubber products aretypically hauled to a dump because there is very little use for themafter they have served their original intended purpose. A limited numberof used tires are utilized in building retaining walls, as guards forprotecting boats and similar things where resistance to weathering isdesirable. Efforts to reclaim scrap vulcanized rubber have primarilyincluded a physical shearing process which is suitable for a rubberwhich can be mixed with asphalt, forming asphalt rubber. However, a fargreater number of tires, hoses, and belts are simply discarded.

During the vulcanization process of rubber, accelerators, promoters,and/or initiators, are used to form large numbers of sulfur crosslinks.After vulcanization, the crosslinked rubber becomes thermoset and cannotbe reformed into other products. Thus, vulcanized rubber productsgenerally cannot be simply melted and recycled into new products. Thesulfur crosslinks which are present in used vulcanized rubber, such astire rubber, are deleterious in a subsequent curing process which usesvulcanized rubber as a component in a new polymer mixture. Formulationsof tire rubber which use more than minor amounts of vulcanized rubberresult in a brittle cured end product unsuitable for many uses such asautomobile or truck tires.

In light of the foregoing, various techniques for devulcanizing rubberhave been developed. For example, in one devulcanization process,vulcanized rubber is placed in an organic solvent to recover variouspolymerized fractions as taught in Butcher, Jr. et al., U.S. Pat. No.5,438,078. Platz, U.S. Pat. No. 5,264,640 teaches taking scrap rubberfrom used tires and regenerating the monomeric chemicals which aresubsequently recovered. This method uses gaseous ozone to break down thecrosslinked structure of the rubber followed by thermal depolymerizationin a reaction chamber. Platz et al., U.S. Pat. No. 5,369,215 teaches asimilar process in which used tire material may be depolymerized underelevated temperatures and at a reduced pressure to recover the monomericcompounds. Myers et al., U.S. Pat. No. 5,602,186 discloses a process fordevulcanizing rubber by desulfurization, comprising the steps ofcontacting vulcanized crumb rubber with a solvent and an alkali metal toform a reaction mixture, heating the reaction mixture in the absence ofoxygen and with mixing to a temperature sufficient to cause the alkalimetal to react with sulfur in the crumb rubber, and maintaining thetemperature below that at which thermal cracking of the rubber occurs,thereby devulcanizing the crumb rubber. Hunt et al., U.S. Pat. No.5,891,926 is directed to a devulcanization process for rubber in whichelevated temperatures and pressures are used to partially devulcanizethe rubber. Thereafter, a solvent 2-butanol is used to extract thedevulcanized rubber from the non-rubber and/or solids component.

Novotny et al., U.S. Pat. No. 4,104,205 discloses a technique fordevulcanizing sulfur-vulcanized elastomer containing polar groups whichcomprises applying a controlled dose of microwave energy of between 915and 2450 MHz and between 41 and 177 watt-hours per pound in an amountsufficient to sever substantially all carbon-sulfur and sulfur-sulfurbonds and insufficient to sever significant amounts of carbon-carbonbonds. Other patents directed to microwave techniques include Lai et al.U.S. Pat. No. 4,440,488; Hayashi et al., U.S. Pat. No. 4,469,817;Ficker, U.S. Pat. No. 4,665,101; and Wicks et al., U.S. Pat. No.6,420,457. In general, the application of microwave energy results inuneven heating of the elastomer. As such, the degree to which theelastomer particles are devulcanized vary within the rubber particle,which is typically most evidenced by different surface and interiorproperties.

Isayev et al., U.S. Pat. No. 5,284,625 discloses a continuous ultrasonicmethod for breaking the carbon-sulfur, sulfur-sulfur and, if desired,the carbon-carbon bonds in a vulcanized elastomer. Through theapplication of certain levels of ultrasonic energy (15 kHz to 50 kHz) inthe presence of pressure and optionally heat, it is reported thatvulcanized rubber can be broken down. Using this process, the rubberbecomes soft, thereby enabling it to be reprocessed and reshaped in amanner similar to that employed with previously uncured elastomers.Other patents directed to ultrasonic devulcanization techniques includeIsayev, U.S. Pat. No. 5,258,414 and Roberson et al., U.S. Pat. No.6,095,440.

Despite the various devulcanization processes known the art, thereremains a need to develop improved devulcanization techniques,especially those that are capable of devulcanizing the rubber particlesin a relatively uniform manner.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a process for devulcanizingcrosslinked elastomer particles. The process comprises (1) providing acomposition comprising vulcanized crosslinked elastomer particles in adevulcanization apparatus, and (2) applying an alternating electricfield to the composition comprising the vulcanized crosslinked elastomerparticles (with optional fresh unvulcanized elastomer, e.g., freshunvulcanized rubber), the alternating electric field having a frequencyand voltage sufficient to devulcanize the crosslinked elastomerparticles. In one aspect, the frequency of the alternating electricfield is in the radiofrequency range, for example about 1 to 100 MHz,and the voltage of the alternating electric field is about 1000 to10,000 V. In another aspect, the crosslinked elastomer particlescomprise a polar vulcanized rubber material. In still another aspect,the crosslinked elastomer particles have a particle size of 6 to 400mesh. In still another aspect, the fresh unvulcanized elastomer or freshunvulcanized rubber comprises at least 10 wt % of the composition.

In one aspect, the composition is placed in a cavity which residesbetween a first electrode (e.g., top electrode) and a second electrode(e.g., bottom electrode). The alternating electric field is generatedbetween the first and second electrodes. The process may be operated inbatch mode or the composition may be continuously fed between the firsttop electrode and the second bottom electrode. In another aspect, thetop electrode is a high voltage electrode having a voltage between 1000and 10,000 V.

In yet another aspect, the alternating electric field is generatedbetween a plurality of top transverse electrode rods and a plurality ofbottom transverse electrode rods. The process may be operated in batchmode or the composition may be continuously fed between the toptransverse electrode rods and the bottom transverse electrode rods.Exemplary conveying devices include a conveyor and roller assembly.

In another aspect, the present invention is directed to a process fordevulcanizing elastomer particles (such as vulcanized rubber) comprisingthe steps of (1) providing a composition comprising vulcanizedcrosslinked elastomer particles in a devulcanization apparatus, (2)applying an alternating electric field to the composition comprising thevulcanized crosslinked elastomer particles (with optional freshunvulcanized elastomer, e.g., fresh unvulcanized rubber), thealternating electric field having a frequency and voltage sufficient todevulcanize the crosslinked elastomer particles, and (3) continuouslyfeeding the composition between a first conveyor and a second conveyor,wherein the alternating electric field is generated between the firstconveyor and the second conveyor, the first conveyor functioning as ahigh voltage electrode and the second conveyor functioning as a groundelectrode. In another aspect, the first conveyor comprises a mainconveying plate having a plurality of plate sections comprised ofelectrically conductive material which are interconnected together. Theplate sections are preferably sized and shaped to permit the mainconveying plate to be conveyed in a loop while maintaining thecross-linked elastomer particles under pressure during thedevulcanization process. In an exemplary aspect, one or more of theplate sections comprise a main body section having a plurality ofprotrusions and recesses such that the protrusions of a first platesection engage corresponding recesses in an adjacent second platesection and wherein a shaft extends through the first and the secondplate sections to secure the first and second plate sections together.The plate sections have one or more holes for engaging teeth of a driverfor driving the first conveyor. In the exemplary aspect, a shaft extendsthrough a plurality of ball bearings, the ball bearings being retainedin a groove formed in a conveyor frame as the first conveyor moves.

In another aspect, the conveyors are operable to compress the vulcanizedelastomer particles (e.g., vulcanized rubber) during the devulcanizationprocess. In one aspect, the composition is compressed to about 50 to5000 psi. During the process, the alternating electric field causesmovement of polar molecules in the vulcanized elastomer particles,whereby friction resulting from the molecular movement translates intoheat throughout the composition.

In still another aspect, a substantially constant voltage is providedbetween the first and second electrodes (e.g., between the first andsecond conveyors). In yet another aspect, the generator is operable togenerate a signal that is substantially a sinusoid having a wavelengthλ, and wherein the composition is positioned between the first andsecond electrodes and adjacent a point on the first electrode that islocated a distance of ¼λ or ¼λ plus a multiple of ½λ from the generatorsuch that the substantially constant voltage is provided between thefirst and second electrodes. In a further aspect, a substantiallyconstant current passes between the first and second electrodes andthrough the composition (e.g., between the first and second conveyors).

In yet another aspect, the present invention is directed to a processfor forming a vulcanized elastomer composition. The process comprises(1) applying an alternating electric field to a first compositioncomprising vulcanized crosslinked elastomer particles such that thealternating electric field has a frequency and voltage sufficient todevulcanize the crosslinked elastomer particles to form a secondcomposition comprising devulcanized elastomer particles, (2) adding acrosslinking agent to the second composition comprising the devulcanizedelastomer particles, and (3) vulcanizing the second composition havingthe crosslinking agent to form the vulcanized elastomer composition. Inone aspect, the frequency of the alternating electric field is about 1to 100 MHz, and the voltage of the alternating electric field is about1000 to 10,000 V.

The first composition may comprise a vulcanized rubber, such as onehaving a particle size of 6 to 400 mesh. The first composition maycomprise vulcanized rubber particles and optionally fresh unvulcanizedrubber. In one aspect, the fresh unvulcanized elastomer (e.g., freshunvulcanized rubber) comprises at least 10 wt % of the firstcomposition. In still another aspect, the present invention comprisesthe step of adding fresh unvulcanized elastomer (e.g., freshunvulcanized rubber) to the second composition prior to the vulcanizingstep. In one aspect, the fresh unvulcanized elastomer (e.g., freshunvulcanized rubber) may comprise at least 10 wt % of the secondcomposition. In still another aspect, the process comprises the step ofadding a filler and/or or vulcanization accelerator to the secondcomposition prior to the vulcanizing step. Suitable crosslinking agentsinclude sulfur or a sulfur donor. In yet another aspect, the vulcanizingstep comprises heating the second composition comprising the crosslinkerand the devulcanized elastomer particles. In another aspect, thevulcanizing step comprises applying an alternating electric field to thesecond composition comprising the crosslinker and the devulcanizedelastomer particles at a voltage and frequency sufficient to vulcanizethe devulcanized elastomer particles with the crosslinking agent.

In still another aspect, the present invention is directed to adevulcanization apparatus for devulcanizing a plurality of cross-linkedelastomer particles. The apparatus comprises a first electrodecomprising a first conveyor and a second electrode comprising a secondconveyor. The first conveyor functions as a high voltage electrode andthe second conveyor functions as a ground electrode. The apparatus alsoincludes a generator operable to apply an alternating electric fieldbetween the electrodes. A devulcanization region is located between thefirst and second conveyors. The first electrode is a high voltageelectrode, preferably having a voltage between 1000 and 10,000 V. Thefirst conveyor preferably comprises a main conveying plate having aplurality of plate sections comprised of electrically conductivematerial which are interconnected together. The plate sections are sizedand shaped to permit the main conveying plate to be conveyed in a loopwhile maintaining the cross-linked elastomer particles under pressure.In an exemplary aspect, the plate sections comprise a main body sectionhaving a plurality of protrusions and recesses such that the protrusionsof a first plate section engage corresponding recesses in an adjacentsecond plate section and wherein a shaft extends through the first andthe second plate sections to secure the first and second plate sectionstogether. Also, the plate sections have one or more holes for engagingteeth of a driver for driving the first conveyor. The shaft extendsthrough a plurality of ball bearings, the ball bearings being retainedin a groove formed in a conveyor frame as the first conveyor moves. Inanother aspect, the frame includes an insulator adjacent to the firstconveyor. The apparatus is preferably capable of compressing vulcanizedelastomer particles in the devulcanization region to about 50 to 5000psi.

Additional aspects of the invention, together with the advantagesappurtenant thereto, will be set forth in part in the description whichfollows, and in part will become apparent to those skilled in the artupon examination of the following, or may be learned from the practiceof the invention. The advantages of the invention may be realized andattained by means of the instrumentalities and combinations particularlypointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram illustrating how vulcanized rubber (orother vulcanized elastomers) may be devulcanized in accordance with thepresent invention and then subsequently revulcanized into useful endproducts.

FIG. 2 is a devulcanization apparatus which employs an alternatingelectric field in accordance with a first embodiment of the presentinvention.

FIG. 3 shows the signal generated by the apparatus of FIG. 2, whereinthe signal is substantially a sinusoid having a wavelength λ and whereina single point (designated as point X) is located at the ¼ wavelengthposition.

FIG. 4 illustrates an alternative electrode configuration for thedevulcanization apparatus of FIG. 2, wherein the voltage between a topelectrode comprising a plurality of tiered plates and a single-platebottom electrode is substantially constant. The electrode configurationis shown without the other components of the devulcanization apparatus.

FIG. 5 shows the peak of the signal generated using the electrodeconfiguration shown in FIG. 4, wherein eight points (designated aspoints A-H) are located at the ¼ wavelength position, and wherein thepeak of the sinusoid of FIG. 3 is superimposed thereon in order toillustrate the differences between the configurations of the topelectrodes of FIGS. 2 and 4.

FIG. 6 is a devulcanization apparatus which employs an alternatingelectric field in accordance with a second embodiment of the presentinvention.

FIG. 7 is a devulcanization apparatus which employs an alternatingelectric field in accordance with a third embodiment of the presentinvention.

FIG. 8A is a devulcanization apparatus which employs an alternatingelectric field in accordance with a fourth embodiment of the presentinvention. For clarity, the frame is not shown, although the groove inthe frame is shown in dashed lines to illustrate the path of conveyance.

FIG. 8B is a top view of a portion of the conveyors shown in FIG. 8Aillustrating the adjacent plate sections held together by shafts. Forclarity, the ball bearings, which are also secured to the shaft, are notshown.

FIG. 8C is an exploded view of a plate section shown in FIG. 8Aillustrating how the shaft and ball bearings are used to secure adjacentplate sections together, and, how the ball bearings are aligned within agroove in the frame.

FIG. 8D is a cross-section of the devulcanization apparatus shown inFIG. 8A illustrating the groove within the frame through which theconveyor rotates.

FIG. 8E illustrates an alternative electrode configuration for thedevulcanization apparatus of FIG. 8A.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention is directed to a process for devulcanizingcrosslinked vulcanized elastomers by applying an alternating electricfield to the vulcanized elastomer. The alternating electric field may beapplied to a wide range of crosslinked vulcanized thermoset elastomers,such as those polymeric networks found in many thermosets. Without beinglimited to the enumerated examples, these vulcanized elastomers includepolyurethanes, epoxy/phenolic resins, epoxy resins, saturated polyesterresins, unsaturated polyester resins, phenolic/formaldehyde resins,rubber, and combinations thereof. Once the elastomer has beendevulcanized, it can be revulcanized into additional useful endproducts. The elastomer may be combined with other materials during thedevulcanization process. For example, it is contemplated that a wholeshoe may be ground, shredded, or otherwise cut into small particles, andthen the rubber from the shoe may be devulcanized in accordance with thepresent invention.

The term “vulcanized” refers to a three-dimensional crosslinkedstructure between the elastomer (e.g., rubber) molecules. Thus, the term“vulcanized rubber” encompasses rubbers having a three-dimensionalcrosslinked structure between rubber molecules. The introduction of thecrosslinked structure may be performed by various crosslinking methodsknown to those skilled in the art, such as those involving sulfurvulcanization, thiuram cure, peroxide vulcanization, and the like.

The term “devulcanized” is used to indicate that certain surface andbulk properties of the crosslinked vulcanized elastomer (e.g.,vulcanized rubber) have been chemically altered by the application of analternating electric field in accordance with the present invention. Ingeneral, the number of mono, di, and polysulfides which formed polymercrosslinks during the initial vulcanization process are reduced by thealternating electric field devulcanization process. As such, theelastomer (e.g., rubber) is referred to here as “devulcanized” though itis understood that some crosslinking may remain in the end“devulcanized” product.

The term “fresh” or “virgin” or “unvulcanized” is used to indicate thatthe elastomer (e.g., rubber) has not been vulcanized.

While the invention will be described in detail below with reference tovarious exemplary embodiments, it should be understood that theinvention is not limited to the specific configuration or methodology ofthese embodiments. In addition, although the exemplary embodiments aredescribed as embodying several different inventive features, one skilledin the art will appreciate that any one of these features could beimplemented without the others in accordance with the invention.

In one exemplary embodiment, the vulcanized elastomer that is to bedevulcanized using an alternating electric field in accordance with thepresent invention is a rubber elastomer. Thus, the present invention isdirected to a process for devulcanizing rubber by applying analternating electric field to the vulcanized rubber. An overview of theprocess 1 is illustrated in FIG. 1. In general, a vulcanized rubbercomposition 2, such as that found in tires, is ground or milled to forma “crumb rubber” composition 3 having a reduced particle size. The crumbrubber composition 3 is then optionally combined with fresh or virginunvulcanized rubber 4 and then devulcanized by applying an alternatingelectric field in accordance with the present invention. The resultingdevulcanized rubber composition 5 may optionally be combined withadditional fresh or virgin unvulcanized rubber 4 and then revulcanizedto form an end product.

The vulcanized rubber composition 2, or more generally the crosslinkedvulcanized elastomer, to be devulcanized by applying an alternatingelectric field in accordance with the present invention may compriseeither polar or non-polar or low-polar rubbers. That is, the vulcanizedrubber may be comprised of rubbers having inherent polarity, forexample, polychloroprene rubber, nitrile rubber, nitrilerubber-poly(vinyl chloride) blends [for example, 30 percent by weightmaximum poly(vinyl chloride), and typically about 20 percent poly(vinylchloride) by weight], bromobutyl rubber, or chlorobutyl rubber. Nearlyall commercial tires are comprised of polar rubbers. Vulcanizednon-polar or low-polar rubbers (for example, polyolefin rubbers (e.g.,ethylene-propylene rubbers, butadiene rubbers, styrene-butadienerubbers), fluorocarbon rubbers like Teflon®, and silicone rubbers may bedevulcanized in accordance with the present invention if the polarityhas been introduced as the result of some other material introduced intothe rubber composition (for example, carbon black). Thus, variousadditives may be added to the rubber composition in order to improve ortune the polarity of the rubber composition as desired. The use ofadditives is generally described in Marc, U.S. Patent Application No.2006/0012083. Other examples of specific vulcanized rubbers which may bedevulcanized by the alternating electric field process includespolyalkenylenes, synthetic elastomers made from monomer of conjugateddienes having from 4 to 10 carbon atoms or interpolymers of said dienes(1) among themselves, or (2) with monomers of vinyl substituted aromatichydrocarbons having from 8 to 12 carbon atoms.

The vulcanized rubber may be obtained from a number of commercialsuppliers, and may include a homogeneous or heterogeneous mixture ofvulcanized rubber from a variety of manufacturers. Preferably, thevulcanized rubber is obtained from used tires. It is readily appreciatedby those having ordinary skill in the art that vulcanized rubberoriginating from used automobile or truck tires will typically encompassproducts originating from many manufacturers and comprising a wideassortment of chemical constituents and compositions. Accordingly, awide variety of different chemicals are expected to be present in thevulcanized rubber.

The vulcanized elastomer (e.g., the vulcanized rubber) is preferably inthe form of small elastomer particles. The particle size of thevulcanized rubber preferably ranges from 6 to 400 mesh (e.g., 10, 40,80, 120, 140, 160, 180, 200, 220, 240, 260, 280, 300, 320, 340, 360,380, 400 mesh, or some range therebetween). The particle size istypically between 10 and 200 mesh, and typically between 30 and 40 mesh.Yet, it is anticipated that smaller particle sizes are most preferred,and that they are typically greater than 100 mesh, and still morepreferably greater than 200 mesh. The particle size of the vulcanizedrubber may be reduced to the preferred range using any suitable meansknown in the art. Typically, the vulcanized rubber is mechanicallyground, milled sheared, or otherwise pulverized into a type of rubberknown as “crumb rubber.” Cryogenic processes for forming crumb rubbermay also be employed. See e.g., Perfido et al. U.S. Pat. No. 5,588,600;and Edson, U.S. Pat. No. 5,927,627. Further, various commercialsuppliers of crumb rubber are known in the art.

Further, as shown in FIG. 1 prior to applying the alternating electricfield to the vulcanized rubber or other vulcanized elastomer, thevulcanized rubber or other vulcanized elastomer may be combined withvirgin or fresh unvulcanized elastomers, such as virgin or freshunvulcanized rubber. Such virgin or fresh unvulcanized rubber may alsobe polar or non-polar, such as natural isoprene rubber,styrene-butadiene rubber, butadiene rubber, nitrile-butadiene rubberetc. Typically, the particles of the vulcanized elastomer (e.g.,vulcanized rubber) and the virgin or fresh unvulcanized elastomers(e.g., unvulcanized rubber) are premixed together. The vulcanized rubbermay be present in the composition to be devulcanized at any suitablelevel—for example, about 10, 20 30, 40, 50, 60, 70, 80, 90, or 100 wt %(or some range therebetween). Preferably, the virgin or fresh rubbercomprises at least 10 wt % of the overall composition. This is oftendetermined by the use and application of the rubber product and thedesired physical properties.

An alternating electric field is then applied to the vulcanized rubber(and optional virgin or fresh unvulcanized rubber) composition in amanner that targets select chemical bonds in the vulcanized rubber. Inparticular, sulfur-carbon and sulfur-sulfur bonds of the vulcanizedrubber are targeted by the alternating electric field. The alternatingfield causes the molecules to vibrate, which creates internal frictionbetween the rubber molecules. The use of an alternating electric fieldto devulcanize crosslinked elastomer materials results in a relativelyeven application of the wave energy throughout the material.

For the devulcanization of vulcanized tire rubber (e.g., crumb rubber),the frequency of the alternating electric field preferably ranges fromabout 1 to 100 MHz, and more preferably from about 10 to 60 MHz. Themost preferred frequencies are the allowed center frequencies forindustrial, scientific, and medical (“ISM”) applications, namely, 27.12MHz or 40.68 MHz. The voltage preferably ranges from about 1000 to10,000 V, more preferably about 3000 to 7000 V, and still morepreferably about 4000 to 6000 V. The exposure time to the alternatingelectric field varies depending upon the voltage applied, the frequencyapplied, the size of the devulcanization apparatus, and the power factorof the rubber, but typically ranges between a few seconds to up to oneminute. For example, application of an alternating electric field at4000 V and 27.12 MHz for 30 seconds is typically sufficient todevulcanize rubber particles comprised of ground tire rubber. If thevoltage is raised to 8000 V but the frequency is maintained at 27.12MHz, the exposure time is decreased, for example to about 7.5 seconds.

Examples are provided below of devulcanization apparatuses forgenerating an alternating electric field between two electrodes whereinthe voltage between the electrodes is substantially constant, which isnecessary to obtain a substantially constant current between theelectrodes and across the vulcanized rubber composition. As used herein,the term “substantially constant voltage” between electrodes, i.e., ahigh voltage electrode and a ground electrode, means that the differencebetween the voltage provided at a point on the high voltage electrodecompared to the voltage provided at each other point on the high voltageelectrode is preferably less than ±10%, more preferably less than ±8%,more preferably less than ±6%, more preferably less than ±4%, and mostpreferably less than ±2%.

FIG. 2 illustrates an exemplary devulcanizing apparatus 10 in accordancewith a first embodiment of the present invention. In general, thevulcanized rubber is placed between two electrodes such that the rubbereffectively becomes the dielectric of a capacitor. An alternatingelectric field generated between the electrodes causes polar moleculesin the rubber to be attracted and repelled by the rapidly changingpolarity of the alternating electric field. The friction resulting fromthis molecular movement causes a degradation of the crosslinking of therubber.

More specifically, devulcanizing apparatus 10 includes a top electrode12 and a bottom electrode 14, both of which are connected to anelectromagnetic energy source or generator 15 operable to generate analternating electric field between the electrodes. For example, the topelectrode 12 may be the high voltage electrode while the bottomelectrode 14 is the ground electrode (or vice versa). The voltagebetween the electrodes is adjustable and varies between differentapplications. Typically, the voltage between the electrodes is in therange of 1000 to 10,000 V, preferably in the range of 3000 to 7000 V,and more preferably in the range of 4000 to 6000 V. The alternatingelectric field is generated at frequencies ranging from about 1 to 100MHz, and is preferably generated at frequencies ranging from about 25 to40 MHz. Most preferably, the alternating electric field is generated ateither 27.12 MHz or 40.68 MHz (both of which are allowed centerfrequencies for industrial, scientific, and medical (ISM) applications).Also included within apparatus 10 are a top mold 16 and a bottom mold 18that together define a devulcanization cavity therebetween. In theillustrated example, a vulcanized rubber composition 20 (with optionalfresh or virgin unvulcanized rubber) is placed within the cavity. Thetop mold 16 and the bottom mold 18 are compressed so as to remove asmuch air as possible from the cavity. An optional vacuum line 25 may beformed in one or both of the top mold 16 or bottom mold 18 in order tofurther remove air from the cavity. In operation, an alternatingelectric field is applied across the rubber composition 20, causing themolecules of the rubber to vibrate and cause devulcanization of therubber. The temperature of the rubber composition typically increases toabout 95 to 175° C., preferably to about 120 to 130° C., during thedevulcanization process.

Generator 15 contains a power tube and LC circuit, or may alternativelycontain solid-state technology. Preferably, generator 15 is tuned toresonate at the selected frequency, which occurs when the inductivereactance balances the capacitive reactance at the selected frequency,as follows:

$\begin{matrix}{f = \frac{1}{2\pi\sqrt{LC}}} & (1)\end{matrix}$where

f=frequency of alternating electric field in hertz

L=inductance in henries

C=capacitance in farads.

As shown in FIG. 3, the signal generated by the apparatus of FIG. 2 issubstantially a sinusoid having a wavelength λ. Preferably, thevulcanized rubber particles are placed between top electrode 12 andbottom electrode 14 and generally centered at a position that is ¼λ or,alternatively, ¼λ plus a multiple of ½λ, (e.g., ¾λ, 1¼λ, etc.), from thepower tube of generator 15. It can be seen that the peak of the sinusoidis located at these positions, which provides the most constant voltage(i.e., the lowest voltage change) on the sinusoid. One skilled in theart will understand that the application of similar voltages across theentire area of the vulcanized rubber particles will result insubstantially even heating of the vulcanized rubber particles.

The wavelength of the sinusoid is expressed as follows:

$\begin{matrix}{\lambda = \frac{c}{f}} & (2)\end{matrix}$where

λ=wavelength of sinusoid in meters

c=speed of light (3×10⁸ m/sec)

f=frequency of alternating electric field in hertz.

Using this equation, the wavelength of a sinusoid for an alternatingelectric field generated at 27.12 MHz is as follows:

$\begin{matrix}{\lambda = {\frac{3 \times 10^{8}}{27.12 \times 10^{6}} = {{11.1\mspace{14mu}{meters}} = {36.3\mspace{14mu}{feet}}}}} & (3)\end{matrix}$Thus, the ¼λ, position is located 9.1 feet from the power tube ofgenerator 15.

Similarly, the wavelength of a sinusoid for an alternating electricfield generated at 40.68 MHz is as follows:

$\begin{matrix}{\lambda = {\frac{3 \times 10^{8}}{40.68 \times 10^{6}} = {{7.5\mspace{14mu}{meters}} = {24.6\mspace{14mu}{feet}}}}} & (4)\end{matrix}$Thus, the ¼λ, position is located 6.15 feet from the power tube ofgenerator 15.

One skilled in the art will understand that the use of a lower frequency(e.g., 27.12 MHz) will provide more consistent voltages betweenelectrodes 12 and 14 due to the longer wavelength λ, of the generatedsignal. However, the use of a higher frequency (e.g., 40.68 MHz) willheat the vulcanized rubber particles at a faster rate. Thus, for anygiven application, the desired frequency may be selected with theseconsiderations in mind. Of course, the surface area of the cavitycontaining the vulcanized rubber particles may dictate the desiredfrequency. For example, if the surface area of the cavity is relativelysmall, it is possible to use a higher frequency (e.g., 40.68 MHz).However, if the surface area of the cavity is relatively larger, it maybe preferable to use a lower frequency (e.g., 27.12 MHz).

As discussed above, apparatus 10 shown in FIG. 2 may be used to applysubstantially constant voltages between electrodes 12 and 14 if thecavity containing the vulcanized rubber particles is placed at or nearthe ¼λ, position (or, alternatively, ¼λ, plus a multiple of ½λ). Withthis electrode configuration, a single point (designated as point X inFIGS. 2 and 3) is located at the ¼ wavelength position (or,alternatively, ¼λ, plus a multiple of ½λ), which corresponds to thehighest voltage on the sinusoid. In order to apply even more consistentvoltages between the electrodes, top electrode 12 may be replaced with atop electrode in which a plurality of points are located at the ¼wavelength position (or, alternatively, ¼λ plus a multiple of ½λ), aswill be described below.

Referring to FIG. 4, a diagram of an exemplary electrode configurationthat may be used in place of the electrode configuration shown in FIG. 2to generate an alternating electric field between two electrodes isdesignated as reference numeral 20. Apparatus 20 includes a topelectrode 22 and a bottom electrode 24, both of which are connected toan energy source or generator 25 operable to generate an alternatingelectric field between the electrodes. It should be understood that theonly difference between the electrode configuration of apparatus 10 asshown in FIG. 2 and the electrode configuration 20 shown in FIG. 4 isthe configuration of the top electrode. In FIG. 2, top electrode 12comprises a single plate. However, in FIG. 4, it can be seen that topelectrode 22 comprises a plurality of electrically connected platesarranged in a tiered configuration. Specifically, top electrode 22includes a main plate 22 a located adjacent the cavity containing thevulcanized rubber particles, which is electrically connected to plates22 b, 22 c, 22 d, and 22 e. Then, plates 22 b and 22 c are electricallyconnected to plate 22 f, and plates 22 d and 22 e are electricallyconnected to plate 22 g. Further, plates 22 f and 22 g are electricallyconnected to plate 22 h, which is electrically connected to the powertube of the generator (or other solid-state supply). As can be seen, inthe illustrated embodiment, the main plate 22 a of top electrode 22 andbottom electrode 24 are positioned proximate to the cavity containingthe vulcanized rubber particles and are sized to extend across thecavity containing the vulcanized rubber particles. Of course, the sizeof the electrodes will vary depending on the surface area of the cavitycontaining the vulcanized rubber particles.

As shown in FIG. 4, points A, B, C, D, E, F, G, and H are evenly spacedalong the length of main plate 22 a, and the power tube of the generatoris designated as point O. The size and positioning of the various platesare chosen such that the distances OA, OB, OC, OD, OE, OF, OG, and OHare the same and, thus, points A, B, C, D, E, F, G, and H are eachlocated at the ¼ wavelength position (or, alternatively, ¼λ, plus amultiple of ½λ). For example, if the selected frequency is 27.12 MHz or40.68 MHz, each of points A, B, C, D, E, F, G, and H would be located9.1 feet or 6.15 feet, respectively, from point O. By contrast, as shownin FIG. 2, only point X is located at the ¼ wavelength position.

FIG. 5 shows the peak of the signal generated by the electrodeconfiguration of FIG. 4, wherein points A, B, C, D, E, F, G, and H arelocated at the ¼ wavelength position (or, alternatively, ¼λ plus amultiple of ½λ). The peak of the sinusoid of FIG. 3, along with point X,is superimposed thereon in order to illustrate the differences betweenthe configurations of top electrode 12 (FIG. 2) and top electrode 22(FIG. 4). As can be seen, point X and points A, B, C, D, E, F, G, and Hare each located at the peak of the sinusoid, which corresponds to thehighest voltage. In effect, the configuration of top electrode 22substantially flattens-out the peak of the sinusoid. As such, topelectrode 22 may be used to apply more consistent voltages betweenelectrodes 22 and 24 as compared to top electrode 12.

Of course, one skilled in the art will understand that top electrode 22is merely an example of an electrode that may be used to provide moreconsistent voltages between the electrodes. Other configurations mayalso be used in which multiple points (i.e., more or fewer points thanthe eight points shown in FIG. 4) are located at the ¼ wavelengthposition (or, alternatively, ¼λ plus a multiple of ½λ). For example,FIG. 8E illustrates an electrode configuration in which four points arelocated at the ¼ wavelength position (or, alternatively, ¼λ, plus amultiple of ½λ), as described below. Stated another way, the topelectrode may comprise any configuration of electrically connectedplates that are sized and positioned such that each of a plurality ofpoints are located the same distance from the power tube of thegenerator.

FIG. 6 illustrates an exemplary devulcanizing apparatus 110 inaccordance with a second embodiment of the present invention. Thedevulcanizing apparatus 110 includes a top electrode 112 and a bottomelectrode 114, both of which are connected to an electromagnetic energysource or generator (not shown) operable to generate an alternatingelectric field between the electrodes. For example, top electrode 112may be the high voltage electrode while bottom electrode 114 is theground electrode (or vice versa). The top electrode 112 may comprise asingle plate (e.g., as generally depicted and described in connectionwith FIG. 2) or may comprise a plurality of electrically connectedplates (e.g., as generally depicted and described in connection withFIG. 4).

Also included within apparatus 110 is a conveyor 118 which is positionedagainst bottom electrode 114 for continuously supplying a compositioncomprising vulcanized rubber particles 120 (e.g., crumb rubberoptionally mixed with fresh or virgin unvulcanized rubber) into theregion between the electrodes. The composition comprising the vulcanizedrubber particles 120 is preferably metered through hopper 130 to one ormore set of rollers 132, 134 to form a generally continuous sheet ofvulcanized rubber particles (and optional fresh or virgin unvulcanizedrubber). The rollers generally compress the vulcanized rubber particles,which assists with removal of air from the rubber. In the illustratedexample, vulcanized rubber (along with the optional virgin or freshunvulcanized rubber) is metered through the rollers 132, 134 onto theconveyor 118 so as to remove as much air as possible from the rubbercomposition 120. In operation, an alternating electric field is appliedacross the rubber composition 120, causing the molecules of the rubberto vibrate and cause devulcanization of the rubber. One or more rollers,presses, or other compacting devices 136, 138 may be used to compressthe sheet material during or after the devulcanization process,preferably while the devulcanized rubber is hot. The temperature of therubber composition typically increases to about 95 to 175° C.,preferably about 120 to 130° C., during the devulcanization process.

FIG. 7 illustrates an exemplary devulcanizing apparatus 210 inaccordance with a third embodiment of the present invention. Thedevulcanizing apparatus 210 includes a plurality of top transverseelectrode rods 212 a-d and a plurality of bottom transverse electroderods 214 a-d. For example, the top transverse electrode rods mayfunction as the high voltage electrodes while the bottom transverseelectrode rods function as the ground electrodes (or vice versa). Thetop transverse electrode rods 212 a-d and the bottom transverseelectrode rods 214 a-d are connected to an electromagnetic energy sourceor generator (not shown) operable to generate an alternating electricfield between the electrode rods. Typically, the voltage between the toptransverse electrode rods 212 a-d and the bottom transverse electroderods 214 a-d is in the range of 1000 to 10,000 V, preferably in therange of 3000 to 7000 V, and more preferably in the range of 4000 to6000 V. The alternating electric field is generated at frequenciesranging from about 1 to 100 MHz, is preferably generated at frequenciesranging from about 25 to 40 MHz, and is more preferably generated ateither 27.12 MHz or 40.68 MHz.

Also included within apparatus 210 is a conveyor 218 which is positionedover the bottom transverse electrode rods 214 a-d for continuouslysupplying a composition comprising vulcanized rubber particles 220(e.g., crumb rubber optionally mixed with virgin or fresh unvulcanizedrubber) into the region between the top and bottom transverse electroderods. The vulcanized rubber particles 220 are preferably metered throughhopper 230 to one or more set of rollers 232, 234 to form a generallycontinuous sheet of vulcanized rubber particles. The rollers generallycompress the composition comprising vulcanized rubber particles 220,which assists with removal of air from the rubber. In the illustratedexample, vulcanized rubber (along with the optional virgin or freshunvulcanized rubber) is metered onto the conveyor 210 and through therollers 232, 234 so as to remove as much air as possible from the rubbercomposition. In operation, an alternating electric field is appliedacross rubber particles 220, causing the molecules of the rubber tovibrate and cause devulcanization of the rubber. One or more rollers,presses, or other compacting devices 236, 238 may be used to compressthe sheet material during or after the devulcanization process,preferably while the devulcanized rubber is hot. The temperature of therubber composition typically increases to about 95 to 175° C.,preferably about 120 to 130° C., during the devulcanization process.

FIGS. 8A to 8E illustrate a devulcanizing apparatus 310 in accordancewith a fourth embodiment of the present invention. Like the priorembodiments, the devulcanization apparatus provides for heating of thevulcanized crosslinked elastomer particles via the application of analternating electric field. The apparatus 310 includes a first conveyor318A and a second conveyor 318B, each having a conveying belt 315A, 315Bwhich is driven by corresponding drivers 319A, 319B. The conveyors 318A,318B are designed for continuously supplying a composition comprisingvulcanized rubber particles 320 (e.g., crumb rubber optionally mixedwith virgin or fresh unvulcanized rubber) into the alternating electricfield. The vulcanized rubber particles 320 are preferably meteredthrough an extruder or hopper 330 to the first and second conveyors318A, 318B to form a generally continuous sheet of vulcanized rubberparticles. The conveyors generally compress the composition comprisingvulcanized rubber particles 320, which assists with removal of air fromthe rubber. The pressure applied to the composition is typically betweenabout 50 to 5000 psi, for example about 50, 100, 200, 300, 400, 500,600, 700, 800, 900, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 4500 orabout 5000 psi or some range therebetween. The conveying belt 315A, 315Bin each conveyor 318A, 318B preferably comprises a material with a lowdielectric constant and a low dissipation factor so that the beltundergoes limited heating during the devulcanization process, and ispreferably comprised of silicone rubber.

In the first conveyor 318A, the main conveying plate 322 a comprises aplurality of plate sections 380 (e.g., 380 a, 380 b, 380 c, 380 d, etc.)which are interconnected together. The plate sections are comprised ofelectrically conductive material, preferably a metal. The plate sectionsare sized and shaped in order to permit the main conveying plate 322 ato be conveyed in a loop while maintaining the vulcanized rubberparticles 320 under pressure during substantially all of thedevulcanization process. Exemplary plate sections are illustrated inFIGS. 8B, 8C, and 8E. Each plate section 380 comprises a main bodysection 382 and a plurality of protrusions 384 and recesses 386. Asgenerally shown in the figures, the protrusions 384 of the first platesection engage corresponding recesses 386 in an adjacent second platesection, and so on. A shaft 389 extending through a plurality of ballbearings 390 and openings 385 in the protrusions 384 is used to securethe adjacent plate sections 380 together. As shown in FIG. 8C, the platesections 380 (e.g., 380 a, 380 b, 380 c, 380 d, etc.) of the mainconveying plate 322 a preferably each have one or more holes 388 forengaging the teeth of the driver. The second conveyor 318B isconstructed in a like manner.

As shown in FIGS. 8A, 8C, 8D, and 8F, the ball bearings 390 are retainedin a groove 350 formed in a conveyor frame 362. The path of theconveyors 318A, 318B corresponds to the path taken as the ball bearingsmove along the looped groove 350 formed in the frame 362 (see FIG. 8A inparticular). The frame includes an insulator 365 adjacent to theconveyor 318A which forms the high voltage electrode (see FIG. 8D inparticular) which is surrounded by an exterior frame (ground shield)369. The frame also includes two side pressure pieces 368 that areadjacent to the vulcanized rubber 320 (or other elastomer). The two sidepressure pieces 368 are fixed within the frame, and together thepressure pieces 368 and the conveyors 318A, 318B maintain the vulcanizedrubber under pressure during the devulcanization process as the rubbermoves along the conveyors. The pressure pieces also control the width ofthe material as it moves along the conveyor through the apparatus. Itwill be appreciated that the pressure pieces 368 may be separateelements or may be part of the insulator 365 such that the pressurepieces and insulator 365 are a unitary material. The remainder of theframe may be comprised of any suitable material, and is preferably anelectrically conductive material, such as a metal. In such a case, theexterior frame also functions as a ground 369. It will be appreciatedthat for clarity purposes, the frame 362 is not illustrated in FIG. 8A;however, the location of the groove 350 holding the ball bearings 390 isincluded in dashed lines in FIG. 8A in order to show the path of theconveyor as it move along the groove in the frame.

In the illustrated example, vulcanized rubber 320 (along with theoptional virgin or fresh unvulcanized rubber) is metered throughdevulcanization apparatus 310 and through the conveyors 318A, 318B so asto remove as much air as possible from the rubber composition. Inoperation, an alternating electric field is applied across rubber 320,causing the molecules of the rubber to vibrate and cause devulcanizationof the rubber. The conveyors compress the rubber composition as it movesalong the conveyors. The temperature of the rubber composition typicallyincreases to about 95 to 175° C., preferably about 120 to 130° C.,during the devulcanization process. The rubber composition exits thedevulcanization apparatus in a state that is largely devulcanized. Theconveyor is preferably variable speed, fixed length, and fixed power.For example, an exemplary conveyor may be operated at a power of about120 KW and speed of about 6 to 8 ft/min. The devulcanization region ispreferably about 18 to 36 inches wide, about 0.25 to 0.5 inches thickand about 6 to 12 feet wide. The throughput is approximately 30 to 40pounds of particles (e.g., rubber) per minute.

Referring to FIG. 8E, details of the exemplary electrode configurationshown in FIG. 8A are provided. In general, the devulcanizing apparatus310 includes an electrode configuration similar to that shown in FIG. 4.With respect to the first conveyor 318A, it can be seen that topelectrode comprises a main conveying plate 322 a (comprised of the platesections 380 as discussed above) electrically connected to plates 322 band 322 c. The moving connection is maintained by a series of conductiveflexible fingers 325 (which may be strips of metal, brushes, and thelike) that extend from plates 322 b, 322 c to contact the moving plate322 a, as shown. Plates 322 b and 322 c are electrically connected toplate 322 d, which is electrically connected to the power tube of thegenerator 360 (or other solid-state supply). The second conveyor 318B(see FIG. 8A) is electrically connected to ground. Thus, the firstconveyor 318A functions as the high voltage electrode while the secondconveyor 318B functions as the ground electrode. Typically, the voltagebetween the first conveyor 318A and the bottom second conveyor 318B isin the range of 1000 to 10,000 V, preferably in the range of 3000 to7000 V, and more preferably in the range of 4000 to 6000 V. Thealternating electric field is generated at frequencies ranging fromabout 1 to 100 MHz, is preferably generated at frequencies ranging fromabout 25 to 40 MHz, and is more preferably generated at either 27.12 MHzor 40.68 MHz.

As can be seen, in the illustrated embodiment, the vulcanizedcrosslinked elastomer particles are placed between the two conveyors318A, 318B such that main conveying plate 322 a of first conveyor 318Aand the bottom conveyor 318B are positioned against the vulcanizedcrosslinked elastomer particles, thereby compressing them. Of course,the size of the conveyors 318A, 318B will vary depending on the surfacearea of the devulcanization region.

As shown in FIG. 8E, points A, B, C, and D are evenly spaced along thelength of main conveying plate 322 a, and the power tube of thegenerator is designated as point O. The size and positioning of thevarious plates are chosen such that the distances OA, OB, OC, and OD arethe same and, thus, points A, B, C, and D are each located at the ¼wavelength position (or, alternatively, ¼λ, plus a multiple of ½λ). Forexample, if the selected frequency is 27.12 MHz or 40.68 MHz, each ofpoints A, B, C, and D would be located 9.1 feet or 6.15 feet,respectively, from point O.

Of course, one skilled in the art will understand that thedevulcanization apparatus shown in FIG. 8A may utilize electrodeconfigurations other than that shown in FIG. 8E. For example, mainconveying plate 322 a could comprise a single plate electrode in whichcase plates 322 b, 322 c and 322 d would not be used (similar to theelectrode configuration generally depicted and described in connectionwith FIG. 2). In this case, a single point would be located at the ¼wavelength position (or, alternatively, ¼λ plus a multiple of ½λ). Also,a plurality of electrically connected plates could be arranged in atiered configuration in which case additional plates (i.e., plates inaddition to plates 322 b, 322 c and 322 d) would be used (similar to theelectrode configuration generally depicted and described in connectionwith FIG. 4). In this case, eight points or more would be located at the¼ wavelength position (or, alternatively, ¼ plus a multiple of ½λ). Itshould be understood that all of these alternative electrodeconfigurations are within the scope of the present invention.

The present invention is also directed to a process for revulcanizingthe devulcanized elastomers formed using the alternating electric fieldas discussed herein. That is, as generally shown in FIG. 1, thedevulcanized elastomers formed by application of the alternatingelectric field may be revulcanized in order to provide new usefularticles. The devulcanized elastomers formed using the alternatingelectric field may also be combined with other polymer stocks (such asvirgin or fresh unvulcanized rubber) and then revulcanized or otherwisecrosslinked.

In an exemplary embodiment, the rubber devulcanized in accordance withthe present invention is revulcanized—either alone or by combining thedevulcanized rubber with virgin or fresh unvulcanized rubber and thensubjecting it to a vulcanization process. The devulcanized rubber formedby application of the alternating electric field may be present in thecomposition to be revulcanized at any suitable level—for example, about10, 20 30, 40, 50, 60, 70, 80, 90, or 100 wt %. Preferably, thedevulcanized rubber comprises at least 10 wt % of the end product. Forcertain articles such as belts, hoses, or shoe treads, it may bepossible to use 100% devulcanized rubber during the revulcanizationprocess.

The devulcanized rubber of the present invention may be used to form avariety of tire tread and tire tread cap rubber compositions such asthose taught in Bauer et al., U.S. Pat. No. 5,378,754, and Burlett etal., U.S. Pat. No. 5,023,301. For instance, the devulcanized rubber maybe blended with a rubber selected from the group consisting ofcis-1,4-polyisoprene (natural or synthetic), cis-1,4-polybutadiene,3,4-polyisoprene, styrene/butadiene copolymers,styrene/isoprene/butadiene terpolymers, butadiene/acrylonitrilecopolymers, isoprene/acrylonitrile copolymers, nitrile/butadienecopolymers and mixtures thereof.

The devulcanized rubber of the present invention may also be combinedwith a suitable filler. Examples of fillers include, but are not limitedto, silica, alumina, diatomaceous earths, titanium dioxide, iron oxide,zinc oxide, magnesium oxide, metal ferrite, and other such oxides;aluminum hydroxide, magnesium hydroxide, and other such hydroxides;calcium carbonate (light and heavy), magnesium carbonate, dolomite,dawsonite, and other such carbonates; calcium sulfate, barium sulfate,ammonium sulfate, calcium sulfite, and other such sulfates and sulfites;talc, mica, clay, glass fiber, calcium silicate, montmorillonite,bentonite, and other such silicates; zinc borate, barium metaborate,aluminum borate, calcium borate, sodium borate, and other such borates;carbon black, graphite, carbon fiber, and other such forms of carbon; aswell as powdered iron, powdered copper, powdered aluminum, zinc flowers,molybdenum sulfate, boron fiber, potassium titanate, and lead titanatezirconate.

Synthetic resins can be utilized as the organic fillers. Examplesinclude powders of alkyd resins, epoxy resins, silicone resins, phenolicresins, polyester, acrylic resins, acetal resins, polyethylene,polyether, polycarbonate, polyamide, polysulfone, polystyrene, polyvinylchloride, fluoro resins, polypropylene, ethylene-vinyl acetatecopolymers, and various other such thermosetting resins or powder ofthermoplastic resins, or powders of copolymers of these resins. Further,other examples of organic fillers which can be utilized include aromaticor aliphatic polyamide fibers, polypropylene fibers, polyester fibers,and aramid fibers.

Antioxidants, UV light stabilizers, and processing oils can be includedas well. Antioxidants include physical protectorants and chemicals whichminimize oxidation. The chemicals include amines, phenolics, andphosphites. Processing oils are usually chosen based on compatibilitywith the rubber and desirable color and/or aging properties. They may beorganic ester plasticizers, mineral oils, vegetable oils, paraffinicoils, naphthenic oils, or other aromatic oils. The rubber compositionmay also include lubricants, antistatic agents, pigments, dyes, flameretardants, or processing aids, which are all well known to thoseskilled in the art.

The methods used to re-vulcanize the elastomer material that waspreviously devulcanized by application of the alternating electric fieldinclude any of those known in the art. In general, the vulcanizationsystem is one suitable for reacting with and crosslinking the elastomermaterial. Depending on the elastomer, suitable crosslinking or curingagents include sulfur, sulfur donors, peroxides, metallic oxides,diamines, bismaleimides, and the like. Powdered sulfur, sulfur flowers,sulfur chloride, deoxygenated sulfur, sediment sulfur, colloidal sulfur,surface-treated sulfur, and the like can be used as the sulfur.Peroxides which may be utilized include, for example, di-t-butylperoxide, t-butylcumyl peroxide, dicumyl peroxide, and other suchdialkyl peroxides, acetyl peroxide, lauroyl peroxide, benzoyl peroxide,and other such diacyl peroxides, methyl ethyl ketone peroxide,cyclohexanone peroxide, 3,3,5-trimethyl cyclohexanone peroxide, methylcyclohexanone peroxide, and other such ketone peroxides, 1,1-bis(t-butylperoxy)cyclohexane, and other such peroxyketals, t-butyl hydroperoxide,cumene hydroperoxide, 1,1,3,3-tetramethyl butyl hydroperoxide,p-menthane hydroperoxide, diisopropylbenzene hydroperoxide,2,5-dimethylhexane-2,5-dihydroperoxide, and other such hydroperoxides,t-butyl peroxyacetate, t-butylperoxy-2-ethyl hexanoate, t-butylperoxybenzoate, t-butyl peroxyisopropyl carbonate, and other such peroxyesters. Examples of maleimide crosslinking agents are m-phenylenebismaleimide (4,4′-m-phenylene bismaleimide), 4,4′-vinylenediphenylbismaleimide, p-phenylene bismaleimide, 4,4′-sulfonyldiphenylbismaleimide, 2,2′-dithiodiphenyl bismaleimide,4,4′-ethylene-bis-oxophenyl bismaleimide, 3,3′-dichloro-4,4′-biphenylbismaleimide, hexamethylene bismaleimide, and 3,6-durine bismaleimide.Zinc oxide may be used alone or in combination with other crosslinkingagents for halogenated rubbers such as bromobutyl rubbers. Resincrosslinking agents can be used. The resins include methylol phenolicresins, brominated phenolic resins, urethane resins etc. The mostpreferred crosslinking agent is sulfur. It is preferably used in anamount of about 0.1 to about 3.0 parts by weight per 100 parts by weightof the rubber polymer. The preferred amount of sulfur is about 1.0 to2.0 wt %.

Various vulcanization accelerators, promoters, and/or initiators can beadded to the vulcanization mixture. Typical vulcanization acceleratorsinclude, but are not necessarily limited to: guanidine typevulcanization accelerators, aldehyde-ammonia type vulcanizationaccelerators, sulphenamide type vulcanization accelerators, thiuram typevulcanization accelerators, xanthate type vulcanization accelerators,aldehyde-amine type vulcanization accelerators, thiazole typevulcanization accelerators, thiourea type vulcanization accelerators,dithiocarbamate type vulcanization accelerators, and mixed types ofthese. Examples of vulcanization accelerators include, for example,tetramethylthiuram disulfide (“TMTD”), tetramethylthiuram monosulfide(“TMTM”), N-oxydiethylene-2-benzothiazolyl sulfenamide (“OBS”),N-cyclohexyl-2-benzothiazyl sulfenamide (“CBS”),N-t-butyl-2-benzothiazyl sulfenamide (“TBBS”), benzothiazyl-2-sulphenemorpholide (“MBS”), N-dicyclohexyl-2-benzothiazyl sulfenamide (“DCBS”),tetramethylthiuram disulfide (“TMTD”); diphenylguanidine (“DPG”),mercaptobenzothiazole (MBT), mercaptobenzothiazole disulfide (“MBTS”),the zinc salt of mercaptobenzothiazole (“ZMBT”), tetramethylthiuramhexasulfide, N,N-diphenylurea, morpholinethiobenzothiazole, zincdi-n-butyl dithiocarbamate, zinc dimethyl dithiocarbamate, and zincflowers. Examples of guanidine type vulcanization accelerators includediphenyl guanidine, triphenyl guanidine, di-ortho-tolyl guanidine,ortho-tolyl biguanide, and diphenyl guanidine phthalate. Examples ofaldehyde-ammonia vulcanization accelerators include hexamethylenediamine and acetaldehyde-ammonia. Examples of thiuram vulcanizationaccelerators include tetramethylthiuram monosulfide, tetramethylthiuramdisulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide, anddipentamethylenethiuram tetrasulfide. Examples of aldehyde-aminevulcanization accelerators include the reaction product of butyraldehydeand aniline and aldehyde-ammonia compounds such as hexamethylene diamineand acetaldehyde-ammonia. Examples of xanthate type vulcanizationaccelerators include, but are not necessarily limited to: sodiumisopropylxanthate, zinc isopropylxanthate, zinc ethylxanthate, zincbutylxanthate, and dibutylxanthate disulfide. Examples of thiazole typevulcanization accelerators include N cyclohexyl-2-benzothiazolesulfenamiden, N-oxydiethylene-2-benzothiazolesulfenamide,N,N-diisopropyl-2-benzothiazole sulfenamide, 2-mercaptobenzothiazole,2-(2,4-dinitrophenyl)mercaptobenzothiazole,2-(2,6-diethyl-4-morpholinothio)benzothiazole, andbenzothiazyldisulfide. Examples of thiourea vulcanization acceleratorsinclude thiocarbanilide, diethylthiourea, dibutylthiourea,trimethylthiourea, and di-ortho-tolyl thiourea. Examples ofdithiocarbamate type vulcanization accelerators include, but arenecessarily limited to: sodium dimethyldithiocarbamate, sodiumdiethyldithiocarbamate, sodium di-n-butyldithiocarbamate, zincethylphenyldithiocarbamate, zinc dimethyldithiocarbamate, zincdiethyldithiocarbamate, zinc di-n-butyldithiocarbamate, zincdibenzylthiocarbamate, zinc N-penta-methylenedithiocarbamate, zincdimethylpentamethylenedithio-carbamate, zinc ethylphenyldithiocarbamate,selenium dimethyldithiocarbamate, selenium diethyldithiocarbamate,tellurium diethyldithiocarbamate, cadmium diethyldithiocarbamate,dimethylammonium dimethyldithiocarbamate, dibutylammoniumdibutyldithiocarbamate, diethylamine diethyldithiocarbamate,N,N′-dimethylcyclohexane salt of dibutyldithiocarbamic acid, pipecolicmethylpentamethylenedithiocarbamate, and the like.

Depending on the particular vulcanization accelerator employed,preferred amounts of accelerator typically range from about 0.1 to about3.0 parts by weight per 100 parts by weight of the rubber polymer. Forexample, an accelerator system comprising about 0.5 to 2 wt % MBTS and0.1 to 0.3 wt % TMTD may be employed for devulcanized crumb rubbers frommost tires during the revulcanization process Vulcanization times maydiffer depending on the crosslinking agent, the vulcanizationaccelerator, and the vulcanization temperature.

A coupling agent may also be used in the present invention. There are nospecific limitations on the coupling agent, and selections suited toobjectives can be made. However, a typical silane coupling agent likeSi69. Examples of other silane coupling agents may includebis(3-triethoxysilylpropyl)tetrasulfide,bis(2-1-triethoxysilylethyl)tetrasulfide,bis(3-trimethoxysilylpropyl)tetrasulfide,bis(2-trimethoxysilylethyl)tetrasulfide,3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane,2-mercaptoethyltrimethoxysilane, 3-nitropropyltrimethoxysilane,3-nitropropyltriethoxysilane, 3-chloropropyltrimethoxysilane,3-chloropropyltriethoxysilane, 2-chloroethyltrimethoxysilane,2-chloroethyltriethoxysilane,3-trimethoxysilylpropyl-N,N′-dimethylthiocarbamoyltetrasulfide,3-triethoxysilylpropyl-N,N′-dimethylthiocarbamoyltetrasulfide,2-triethoxysilylethyl-N,N-dimethylthiocarbamoyltetrasulfide,3-trimethoxysilylpropylbenzothiazoletetrasulfide,3-triethoxysilylpropylbenzothiazoletetrasulfide,3-triethoxysilylpropylmethacrylatemono sulfide, and3-trimthoxysilylpropylmethacrylayemono sulfide.

In a preferred aspect, the elastomers that have been devulcanized byapplication of the alternating electric field are revulcanized using oneof two preferred methods. The first involves heating the devulcanizedelastomers (preferably with virgin or fresh unvulcanized rubber) to asuitable temperature for a suitable time. Typically, heating in an ovenat 150 to 180° C. for 5 to 25 minutes will result in sufficientrevulcanization. Second, the elastomers may be heated in a radiofrequency mold, such as the one described in Marc, U.S. Published PatentApplication No. 2009/236030. The exposure time to the alternatingelectric field varies depending upon the voltage applied, but typicallyranges between several seconds to up to two minutes. Thus, the exposuretime is generally longer than that utilized to devulcanize theelastomer. For example, application of 8000 V for 10 to 50 seconds at27.12 MHz is typically sufficient to vulcanize a rubber compositionabout 0.1 to 1.0 inch thick having a sulfur crosslinking agent addedthereto.

It will be appreciated that while Marc, U.S. Published PatentApplication No. 2009/0236030 describes a process for vulcanizing rubberusing an alternating electric field, the present invention describedherein is one in which vulcanized rubber is devulcanized using analternating electric field. Vulcanization typically requires much highertemperatures (on the order of 150 to 220° C., and typically about 160 to190° C.) and the addition of a crosslinking agent—although theoreticallysome crosslinking may occur by virtue of residual sulfur in thedevulcanized rubber.

The examples below are intended to illustrate the present invention. Thedescriptions in no way limit the scope of the present invention. In theexamples, the properties of the combination of vulcanized rubber samplesare evaluated as follows:

Coefficient of friction: Measured using an aluminum sheet.

Abrasion Weight Loss: Loss of weight after 4000 revolutions on a diamondwheel abrader.

Tensile Strength: Measurements were carried out in accordance with ASTMStandard D412, test method A.

Elongation at Break: Measured as a percentage value according to ASTMstandard D412, test method A.

Shore Hardness: Measurements carried out in accordance with ASTMStandard D2240.

Modulus 300%: Measurements were carried out in accordance with ASTM

Standard D412, test method A.

EXAMPLE 1 Truck and Car Tire Rubber

In this example, the physical parameters of vulcanized tire rubberderived from unground tires are shown in column 1 of Table 1. Forcomparison, vulcanized ground tire rubber (“VGTR”) obtained from amixture of truck and car tires from Quest Recycling Services (Concordia,Kans.) was devulcanized by applying an alternating electric field havinga voltage of about 4000 to 6000 V at a frequency of 27.12 MHz for about30 to 50 seconds. The VGTR likely comprised a mixture ofnitrile-butadiene rubber and styrene-butadiene rubber in unknown ratioshaving a 30 mesh particle size. The VGTR was placed in a mold thatmeasures 8×12 inches and was about ¼-inch thick and generally configuredas shown in FIG. 2.

Following devulcanization, the devulcanized ground tire rubber (“DGTR”)was combined with varying amounts of virgin or fresh unvulcanized rubber(“FR”) and then vulcanized. The FR comprised a mixture of isoprene,styrene butadiene, polybutadiene, filler, zinc oxide, stearic acid, oil,sulfur, and accelerator. About 2.5 wt % (based on the weight of therubber) of sulfur, 1.75% MBTS, and 0.3% TMTD was added to the varyingDGTR/FR compositions and then the composition was heated in an oven.During vulcanization, the temperature of the rubber composition reachedtemperature of about 160 to 190° C. The properties of the revulcanizedcompositions from the VGTR that has been devulcanized to form DGTR usingthe alternating electric field are shown in columns 2-7 of Table 1.

TABLE 1 Physical Properties of VGTR and Revulcanized DGTR/FR VulcanizedTire 60% 70% 80% 90% Rubber (before DGTR DGTR DGTR DGTR 100% Propertiesbeing ground) 100% FR 40% FR 30% FR 20% FR 10% FR DGTR Coefficient ofFriction 0.69 0.75 0.84 0.86 0.87 0.89 0.91 Abrasion Weight loss (g)0.31 0.02 0.23 0.24 0.25 0.31 0.34 Y 0.06Y 0.74Y 0.77Y 0.80Y Y 1.1YTensile Strength (kg/cm²) 94 113 88 88 81 55 58 Z 1.2Z 0.94Z 0.94Z 0.86Z0.58Z 0.62Z Elongation at break (%) 375 450 330 325 300 250 185 HardnessShore A 68 69 70 71 74 76 81

EXAMPLE 2 Shoe Rubber

In this example, the physical parameters of vulcanized shoe sole rubberderived from composition comprising virgin or fresh shoe sole rubber(“FSSR”) and scrap devulcanized ground shoe sole rubber (“DGSSR”) areshown in Table 2. Vulcanized ground shoe sole rubber (“VGSSR”) wasground into small particles (40 mesh) and devulcanized by applying analternating electric field having a voltage of about 4000 to 6000 V at afrequency of 27.12 MHz for about 30 to 50 seconds. The VGSSR was placedin a mold that measures 8×12 inches and was about ⅛-inch thick andgenerally configured as shown in FIG. 2.

Following devulcanization, the DGSSR was combined with varying amountsof FSSR and then vulcanized. More specifically, about 1.1 wt % (based onthe weight of the rubber) of sulfur, 0.8% MBTS, and 0.12% TMTD was addedto the varying DGSSR/FSSR compositions and then the composition washeated in an oven. During vulcanization, the temperature of the rubbercomposition reached a temperature of about 160° C. over a period of sixminutes. The properties of the revulcanized compositions from the VGSSRthat has been devulcanized to form DGSSR using the alternating electricfield are shown in Table 2.

TABLE 2 Physical Properties of VGSSR and Revulcanized DGSSR/FSSR 100%50% DGSSR 70% DGSSR 100% Properties FSSR 50% FSSR 30% FSSR DGSSRCoefficient of Friction 0.800 0.867 0.800 0.734 Abrasion Weight loss (g)0.23 0.21 0.19 0.17 Tensile Strength 159 130 95 61 (kg/cm²) Elongationat break (%) 790 575 500 325 Hardness Shore A 63 66 67 65 Modulus 300%(kg/cm²) 51 47 38 NA

EXAMPLE 3 Passenger Car Tire Rubber

In this example, the physical parameters of vulcanized passenger cartire rubber derived from composition comprising virgin or fresh rubber(“FR”) and devulcanized ground passenger car tire rubber (“DGCTR”) areshown in Table 3. The vulcanized ground passenger car tire rubber(“VGCTR”) from Lehigh Technologies (Tucker, Ga.) had a particle size of40 mesh, and was devulcanized by applying an alternating electric fieldhaving a voltage of about 4000 to 6000 V at a frequency of 27.12 MHz forabout 30 to 50 seconds. The VGCTR was placed in a mold that measures8×12 inches and was about ¼-inch thick and generally configured as shownin FIG. 2.

Following devulcanization, the devulcanized ground tire rubber (“DGCTR”)was combined with varying amounts of virgin or fresh unvulcanized rubber(“FR”) and then vulcanized. The FR comprised a mixture of isoprene,styrene butadiene, polybutadiene, filler, zinc oxide, stearic acid, oil,sulfur, and accelerator. More specifically, about 2.5 wt % (based on theweight of the rubber) of sulfur, 1.75% MBTS, and 0.3% TMTD was added tothe varying DGCTR/FR compositions and then the composition was heated inan oven. During vulcanization, the temperature of the rubber compositionreached a temperature of about 160 to 190° C. The properties therevulcanized compositions from the VGCTR that has been devulcanized toform DGCTR using the alternating electric field are shown in Table 3.

TABLE 3 Physical Properties of Revulcanized DGCTR/FR 70% DGCTR 70% DGCTR100% (Mesh 40) (Mesh 105) Properties FR 30% FR 30% FR Coefficient ofFriction .75 .734 1.16 Abrasion Weight Loss (g) .02 NA .07 Tensile(kg/cm²) 113 129.5 125 Elongation at Break (%) 450% 350% 440% HardnessShore A 69 73 72

From the foregoing it will be seen that this invention is one welladapted to attain all ends and objectives herein-above set forth,together with the other advantages which are obvious and which areinherent to the invention. Since many possible embodiments may be madeof the invention without departing from the scope thereof, it is to beunderstood that all matters herein set forth or shown in theaccompanying drawings are to be interpreted as illustrative, and not ina limiting sense. While specific embodiments have been shown anddiscussed, various modifications may of course be made, and theinvention is not limited to the specific forms or arrangement of partsand steps described herein, except insofar as such limitations areincluded in the following claims. Further, it will be understood thatcertain features and subcombinations are of utility and may be employedwithout reference to other features and subcombinations. This iscontemplated by and is within the scope of the claims.

What is claimed and desired to be secured by Letters Patent is asfollows:
 1. A devulcanization apparatus for devulcanizing a plurality ofcross-linked elastomer particles, comprising: a first conveyorfunctioning as a high voltage electrode, wherein the first conveyorcomprises a first main conveying plate having a plurality of platesections comprised of a first electrically conductive material which areinterconnected together; a second conveyor functioning as a groundelectrode, wherein the second conveyor comprises a second main conveyingplate having a plurality of plate sections comprised of a secondelectrically conductive material which are interconnected together; agenerator operable to apply an alternating electric field between saidfirst and second conveyors; and a devulcanization region between saidfirst and second conveyors in which said cross-linked elastomerparticles are placed.
 2. The apparatus of claim 1 wherein the voltageapplied to said first conveyor is between 1000 V and 10,000 V.
 3. Theapparatus of claim 1 wherein said plate sections of said first andsecond main conveying plates are sized and shaped to permit the firstand second main conveying plates to be conveyed in a loop whilemaintaining the cross-linked elastomer particles under pressure.
 4. Theapparatus of claim 1 wherein each of said plate sections comprises amain body section having a plurality of protrusions and recesses suchthat the protrusions of a first plate section engage correspondingrecesses in an adjacent second plate section and wherein a shaft extendsthrough said first and said second plate sections to secure the firstand second plate sections together.
 5. The apparatus of claim 1 whereineach of the plate sections has one or more holes for engaging teeth of adriver for driving the first and second conveyors.
 6. The apparatus ofclaim 4 wherein said shaft extends through a plurality of ball bearings,said ball bearings being retained in a groove formed in a conveyor frameas the first conveyor moves.
 7. The apparatus of claim 6 wherein saidconveyor frame includes an insulator adjacent to said first conveyor. 8.The apparatus of claim 1 wherein said first and second conveyors arecapable of compressing vulcanized elastomer particles in saiddevulcanization region to about 50 to 5000 psi.
 9. The apparatus ofclaim 1 wherein said generator is operable to generate a signal that issubstantially a sinusoid having a wavelength λ, and wherein saidplurality of cross-linked elastomer particles are positioned betweensaid first and second conveyors and adjacent a point on said firstconveyor that is located a distance of ¼λ or ¼λ plus a multiple of ½λfrom said generator such that a substantially constant voltage isprovided between said first and second conveyors.
 10. The apparatus ofclaim 1 wherein a substantially constant current passes between saidfirst and second conveyors.